Are bloggers parasites? That's the question of the day in the navel-gazing world of the blogosphere. Robert Niles, the editor of the Online Journalism Review, recently decried what he sees as a tendency by journalists to characterise blogs as "a 'parasitic' medium" that feeds off the work of traditional newspapers and magazines. He calls the charge "a poorly informed insult of many hard-working Web publishers who are doing fresh, informative and original work".

Maybe so, but Niles's protestations notwithstanding, blogs are largely parasitic. Yes, a handful of bloggers do original reporting, usually on highly specialised topics, but most simply react to the news of the day. The blogosphere, as others have pointed out, acts as a kind of global echo chamber. An idea gets swatted around like a ping-pong ball for a few hours until a fresh one takes its place.

But is that really so bad? I used to think of blogging's reactive nature as a flaw in the medium. I've changed my mind, though. I've come to believe that being a literary parasite is no bad thing. I'd argue, in fact, that parasitism is blogging's most distinctive and probably its most valuable feature.

Bloggers blog for a host of reasons, but what sets blogging apart as a literary form is that it offers a writer an easy way to document his or her responses to their day-to-day reading. The constant flow of text through the eye and mind is a characteristic of many people's lives, but it has never been possible before to capture the experience so thoroughly and with such immediacy as it can be through blogging. Diaries come closest, but they're private, and I'd argue that they place more distance between the act of reading and the act of writing about reading.

The reactive, or parasitical, quality of blogging defined the form from the start. Blogs, after all, began as logs, chronological catalogues of what web surfers discovered in their daily perambulations around the internet. Many of the most accomplished and venerable bloggers continue to write in this form.

The least interesting blogs are the ones that simply replicate existing journalistic forms such as news articles, company profiles or product reviews. They can be very useful, and they can certainly be very popular, but they're blogs in a technical sense only.

In his new book, The Ghost Map, Steven Johnson describes how London's teeming underclass economy in the mid-19th century was built almost entirely on scavenging. The poor were parasites who sustained themselves by collecting the leavings of other Londoners - rags, bones, bits of coal and wood, faeces - and, with remarkable enterprise, transforming them into cash. Our natural instinct, Johnson writes, is "to fulminate against a system that allowed so many thousands to eke out a living by foraging through human waste". But our outrage, he suggests, should "be accompanied by a measure of wonder and respect". After all, "this itinerant underclass managed to conjure up an entire system for processing and sorting the waste generated by 2m people". Without them, London would have been swallowed up in its own filth.

Johnson goes on to draw an analogy between these human waste-recyclers and their microscopic counterparts, bacteria. "Without the bacteria-driven processes of decomposition, the Earth would have been overrun by offal and carcasses eons ago," he reminds us. "If the bacteria disappeared overnight, all life on the planet would be extinguished within a matter of years." Bloggers do similarly useful work. In fact, the blogosphere may best be thought of as a vast digestive tract, breaking down the news of the day into ever finer particles of meaning (and ever more concentrated toxins).

It's worth remembering that, in a literary context, another word for "parasitic" is "critical". Blogging is, at its essence, a critical form, a means of recycling other writings to ensure that every molecule of sense, whether real or imagined, is distilled and consumed.

So if someone wants to call my blog parasitic, or even bacterial, that's fine with me. I'll consider it not an insult, but a compliment.

· Nicholas Carr is the author of Does IT Matter? He scavenges at roughtype.com

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